Is it safe for me to use the 'processed' option (on Shapeways) to smooth the surface, if I'm printing the tapped version? I'm afraid it might affect the threads.
Also, what units should I be using on the site?
I don't really know -- it would have more to do with their process than with the model. I don't see "processed" as an option but I do see "smooth," which is them using a chemical vapor to slightly melt the surface. I would think it might well melt the threads.
Great point on the units. The model is natively in mm. If you upload using the 3MF format, it will automatically use the correct units.
I feel like Shapeways has raised their prices a bit since the last time I quoted this design. Like everyone else, I suppose.